It has become increasingly important in a number of various and diverse industries to have access to ultra high purity process gas supply systems. For example, in the semi-conductor industry, as integrated circuits or similar components become smaller in size, in the micron or submicron ranges, thin film etching processes require gases of ultra high purity. Without a reliable reaction environment for film making and etching, repeatable results are not always achievable.
Although there is ample supply of UHP gases, it is common to experience the introduction of ambient contaminants during cylinder change-out. As noted, these contaminants and their reaction products with process gases can significantly degrade the performance of any UHP gas system.
The problems discussed above are certainly well known and appreciated. In fact, it is common practice to purge ambient contaminants after cylinder change-out and before opening the cylinder valve. In this regard, reference is made to FIG. 1 which depicts three known purging techniques, namely cross-purge, deep purge and vacuum purge.
FIG. 1(A) depicts a typical cross-purge configuration whereby UHP gas cylinder 1 containing high purity gas at high pressure is fed through control valve/cylinder connection 2. Typically, gas travels through a process line depicted by "pigtail" 6 for feeding a process gas through valve 5. Purging takes place by closing valve 2 and applying the purge gas through valve 3 which is exhausted through exhaust valve 4. Although this process provides for some contaminant removal, too much "dead volume" is left in "pigtail" 6 and cylinder connection 2 to adequately remove sufficient contamination.
FIG. 1(B) depicts what is known as a deep purge procedure whereby UHP gas cylinder 11 is functionally attached to connector 12 which further embodies valve 13 for the introduction of purge gas through line 16 and exhaust valve 14. Deep purge provides improved contaminant removal in light of purge gas introduction through valve 13 at connector 12. As such, deep purge eliminates the "dead volume" in the "pigtail" and particularly in the cylinder connection itself. By maintaining process valve 15 in a closed condition, purging is generally accomplished by several pressure cycles, that is, by opening and closing exhaust valve 14, line 16 can be pressurized and depressurized. In doing so, deep purge is effective in removing contaminants in the "dead volume" of the cylinder connection but is not effective in removing contaminants adsorbed on the surface of components. Another problem with the deep purge process is that it is not possible to protect the cylinder valve connection from ambient contaminants by flowing an inert gas through the connecting pieces during cylinder change out.
The present state of the art purge techniques are shown in FIG. 1(C). In this instance, UHP gas cylinder 21 feeds gas to line 26. However, purge gas introduced through valve 22 is employed in conjunction with vacuum generator 23. Purge gas is admitted to line 26 while both vacuum valve 24 and process valve 25 remain in a closed condition. After purge gas pressure buildup, valve 24 is opened and vacuum generator 23 employed to exhaust the purge gas from line 26.
Even the configuration depicted in FIG. 1(C) is not without its drawbacks. Specifically, it has been found that vacuum generator 23 is simply too far from cylinder 21 and its cylinder connection where contamination occurs. The effectiveness of vacuum purge degrades significantly with distance especially for adsorbing species such as moisture. In addition, a vacuum generator increases costs as well as the physical dimension of the purge unit contributing to the complication of system operation.
It is noted that most semi-conductor processing gases, such as those recited in U.S. Pat. No. 4,917,136 are introduced through flow restricting orifices installed in cylinder valve assemblies for safety purposes. Deep purge, cross purge and vacuum purge will not effectively remove contaminants from such flow restricting orifices.
It is thus an object of the present invention to provide a device for purging cylinder valve assemblies more effectively than those employed by the prior art as discussed above.
It is yet a further object of the present invention to provide a device for the removal of contaminants from a gas cylinder valve assembly having fewer parts and being less complex than devices used for the same purpose as discussed above.